Solid state lasers are generally operative either in a normal or free-lasing mode or in a switched or "Q" mode. In the normal mode, a pulse envelope is produced which may typically be of about 1 microsecond in duration, consisting of 15 to 30 spike, each spike being of 30 to 100 nanoseconds duration. In the switched or "Q" mode, a single pulse is produced, typically of 20 to 100 nanoseconds duration. In the laser art, "Q" is defined as the ratio of energy stored in the laser cavity to energy loss per round trip passage of light. In the switched or "Q" mode, means such as a Pockels Cell are provided for controlling the Q which may be kept at a low value while energy is stored by optical pumping. Then the Q is switched to a high value to release the stored energy, a pulse of very high intensity and short duration being thereby produced. It has been recognized that the generation of such a pulse is highly advantageous for certain applications and considerable research and development efforts have been directed toward perfecting the switched or "Q" mode technique.
This invention is directed to the generation of a "stretched" pulse of long duration but of low intensity which has not been generally recognized as being highly desirable but which has been investigated and used. In certain proposed systems, the light is modulated through passive techniques, as by using a non-linear doubling crystal or by lengthening the oscillator cavity. Such passive techniques have serious limitations in that they produce light pulses whose time variation is either gaussian or extremely asymmetric. In other proposed systems, the light is modulated through active techniques, as by using a feedback loop in which a portion of the output of a laser beam is sampled by a photodiode and used to control a Pockels Cell in the oscillator cavity. For example, in an article entitled "Controlled Timewise Redistribution of Laser Energy" by Emilio Panarella and Lorne L.T. Bradley, IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-11, No. 5, May 1975, an electronic feedback circuit is described for a Nd:glass high-power laser system for converting the spikey light emission into a smooth near-rectangular output of adjustable intensity and duration. The voltage at one electrode of a Pockels Cell is directly controlled by a photodiode which is connected in series with an adjustable resistance across a -3KV voltage source, a constant bias voltage being applied to the other electrode. Another active technique is described in an article entitled "Pulse Stretching and Shape Control by Compound Feedback in a Q-SWitched Ruby Laser" by R. V. LovBerlg, et al. IEEE J. Quantum Electronics, Vol. OE-11, No 1, pp. 17-21, January 1975.